1. Field of the Invention
This invention relates to a method of and a system for detecting a failure of an automatic transmission for an automobile, and, more particularly, to a method of and a system for making a decision on a failure of an automatic transmission in gear shift to an intended gear.
2. Description of Related Art
A multiple speed transmission gear mechanism of an automatic transmission equipped with a torque converter is typically connected to an engine through the torque converter. Gear shifts of the automatic transmission are completed by selectively actuating a plurality of electromagnetic solenoids to hydraulically lock and unlock friction coupling elements, such as brakes and clutches, so as to create desired power transmission paths of the multiple transmission gear mechanism. Specifically, control signals are generated and selectively actuate the electromagnetic solenoids according to predetermined gear shift patterns selected on the basis of driving conditions.
Automatic transmissions of this kind possibly encounter failures that an expected or target gear is not created for some reasons or others. A decision on an occurrence of such a shift failure, which is always one of matters of importance for safety drive, can be made by a comparison between a theoretical turbine speed, theoretically determined based on a vehicle speed and a target gear, and an actual turbine speed. If the automatic transmissions are equipped with turbine speed sensors, the failure decision is easy and accurate. However, in some cases where the automatic transmission is not equipped with a turbine speed sensor, a matter of great importance is how to decide an occurrence of a shift failure and how accurately the failure decision is made.
One of approaches to making a failure decision is to estimate a turbine speed on the basis of an engine speed and a speed ratio e of the torque converter. The torque converter speed ratio e, i.e. a speed ratio between a turbine speed and a pump speed, is obtained from a speed ratio map established in relation to input torque coefficient T as a parameter which is an engine output torque T divided by a square of the engine speed NE. In this instance, the engine output torque T is obtained from a torque map established in relation to engine throttle opening TVO and engine speed NE. Such a technique is known from, for instance, Japanese Unexamined Patent Publication No. 6(1994)-331020.
This technique taught by the Japanese Unexamined Patent Publication No. 6(1994)-331020 does, however, not always provide an accurate and reliable decision on an occurrence of a failure in gear shift. This is because the estimate for a turbine speed necessities a number of mathematical calculations and experiments errors in various parameters, such as the temperature and pressure of an operating oil in the multiple transmission gear mechanism.